In aircrafts, it is necessary because of high safety requirements to check the state of the structure of an aircraft. For example, if cracks or fractures arise in the structure and/or in a structure surface, this must be recognized and repair measures must be initiated if necessary.
One possibility for examining crack formation in components is offered by, in addition to conventional nondestructive testing (NDT), the structural health monitoring (SHM) method, which is known in professional circles. The structural health monitoring method is understood as monitoring of components using permanently integrated sensors. In contrast, in conventional NDT, the sensors are removed from the component surface again after the testing. Because of the permanently integrated sensors, more rapid structural monitoring is achieved with the aid of SHM than with conventional NDT, which results in reduced maintenance costs and increased availability of an aircraft.
One technology of SHM is comparative vacuum measurement (CVM) method, which is known in professional circles. A sensor substrate or a sensor film has various air and vacuum channels, the so-called galleries, the air galleries having an atmospheric pressure and the vacuum galleries having a partial vacuum or a vacuum atmosphere. The sensor film is glued onto a component to be tested. If a crack arises on the surface below the CVM sensor during the operation of the aircraft structure, air flows out of the air galleries via the crack into the vacuum channel. The change of the pressure differential between the vacuum gallery and the air gallery resulting therefrom is measured as a signal carrier for the crack detection.
Up to this point, the air and vacuum channels have been produced on the sensor film using replica methods (casting methods) or using laser lithography. In the replica methods, a negative mold is firstly produced, which carries the vacuum and air galleries. A monomeric or oligomeric reaction mixture is then poured into the mold and hardened therein. The resulting positive is the CVM sensor—a plastic film which carries the vacuum and air galleries.
DE 10 2004 057 290 A1 describes an introduction of the vacuum and air galleries with the aid of laser lithography into the sensor substrate or the sensor film. The plastic or the polymer is vaporized by the laser beam because of the local heat introduction. The desired gallery pattern is introduced by appropriate lateral movement of the laser. The depth of the surface removal is controlled by the ratio of feed rate and laser intensity and by the number of laser passes. It is extremely difficult to set the required laser strength and the exact feed to set the depth of the galleries exactly. Multiple passes are frequently necessary to obtain the desired gallery shapes.